I've tried interacting with every item in all three locations (wall, truck, RV), but can't find anything that's not dependant on getting the icepick, which I'll presumably use to kill the zombie near the car.

Basically, when you reach at the motel turn to the right side and on the floor there is a pillow. Grab it. When you are by the truck you just need to go and kill the zombie sitting on the floor and, open the car, and get the spark-plug. When you have done that pull that thing by the steering wheel; there should be a long thing that sticks out, pull that, exit the car, and push the car into the zombie by the wall.

Then you should be by the truck again. Stand up and try to break the window again, this time do it with the spark plug. When you start to do it, the man with you will give you and idea and then break it with the remains of the spark-plug.

go back to the point where you started, look beside the brick wall towards the RV. Instead of running for the RV, grab the pillow on the ground next to you. Go back to behind the truck, and go to run over to the walker sitting on the ground next to the car. When you run up to him, you will kill him. Then open the passenger door to the car, grab the spark plug on the seat. Now before you push the car, run back to the truck and use the spark plug to smash open the window. Then voila you have the ice pick!